DebugInfo: PR14728: TLS support

Based on GCC's output for TLS variables (OP_constNu, x@dtpoff,
OP_lo_user), this implements debug info support for TLS in ELF. Verified
that this output is correct/sufficient on Linux (using gold - if you're
using binutils-ld, you'll need something with the fix for
http://sourceware.org/bugzilla/show_bug.cgi?id=15685 in it).

Support on non-ELF is sort of "arbitrary" at the moment - if Apple folks
want to discuss (or just go ahead & implement) how this should work in
MachO, etc, I'm open.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185203 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetLoweringObjectFile.h

@@ -138,6 +138,10 @@ public:
     return StaticDtorSection;
   }
 
+  /// \brief Create a symbol reference to describe the given TLS variable when
+  /// emitting the address in debug info.
+  virtual const MCSymbolRefExpr *getDebugThreadLocalSymbol(const MCSymbol *Sym) const;
+
 protected:
   virtual const MCSection *
   SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,